In many applications of monomode optical fibers--including coherent communication systems,--it is important for the optical signal which propagates along a fiber to maintain its polarization characteristics constant and stable also with respect to a reference rigidly connected to the fibre. A way to obtain this condition is to use, as a transmission medium, a so-called polarization-maintaining fiber.
A polarization-maintaining fiber is essentially a birefringent element, i.e. an element with different refractive indices in two directions orthogonal with respect to each other and to the axis of the fibre. In the production of these fibers, in particular for use in telecommunications, two techniques are generally employed to obtained birefringence in a naturally isotropic material, such as glass. The first technique involves the production of fibers whose cores do not have axial symmetry, e.g. is elliptical or rectangular or is associated with external elements which alter the distribution of the guided electromagnetic field. The second technique involves the fabrication of fibers whose cores are kept in a condition of transverse mechanical stress.
The main drawback of these techniques is that they require dedicated manufacturing processes, which cannot be used to manufacture conventional fibers. It would, instead, be desirable to use a technique allowing induction of birefringence in the course of a manufacturing process which can also be used for conventional fibers.
The paper "Single Pulse Bragg Gratings Written during Fiber Drawing" by L. Dong et al., Electronics Letters, Aug. 19, 1993, Vol. 29, No. 17, pp. 1577-1578, describes a method of fabricating gratings with periodic variation of the refractive index inside the core of a fiber, while the fiber is being drawn. The method produces refractive index alterations in the germania doped silica, utilizing a UV beam, of such energy as to break the Ge--Ge bridges present inside the doped silica matrix. In particular, according to the cited paper, the fiber is irradiated, in an area immediately upstream of the devices applying the coating, with a wave front resulting from the interference of two parts of a same pulse which have been sent along the two branches of an interferometer and which recombine at the core of the fiber. A periodic modulation of the refractive index of the core is thus obtained, which brings about the periodic variation in reflectivity necessary to fabricate a grating. Instead of the interferometer, a phase mask can be used, as described for instance by K. O. Hill et al. in "Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask", Applied Physics Letters, Vol. 62, No. 10, Mar. 8, 1993.
Although the method described in the paper by L. Dong is oriented to the fabrication of a sequence of discrete components, the same technique could be used, in theory, to realise a grating distributed along a whole fiber, which would thus be a polarization-maintaining fiber, since the profile of the refractive index is different along different axes of a given section of the fiber.
However, such a solution is difficult to realise in practice. Since the grating is "written" into the fiber in a sequence of steps, the grating could present discontinuities. Moreover, the fabrication of a grating presents problems as to the correct alignment of the means which bring about the modulation, especially if it is desired to alter the profile of the refractive index of the original matrix according to multiple, different axes. Furthermore, a grating is an intrinsically wavelength-selective structure, which limits the flexibility of employment of the fibers thus obtained.